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%I #25 Jul 08 2021 06:21:39
%S 0,1,3,4,6,70,73,109,237,264,337,496,1029,1077,1254,1288,2049,3606,
%T 5035,9163,35899
%N Numbers k such that 3*R_(k+2) - 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that (94*10^k - 1)/3 is prime.
%C Terms from Kamada.
%C a(22) > 250000.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/abaaa.htm">Near-repdigit numbers of the form ABAA...AA</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/31333.htm#prime">Prime numbers of the form 3133...33</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=3, 3*R_5 - 2*10^3 = 33333 - 2000 = 31333 which is prime.
%t Select[Range[0, 250000], PrimeQ[(94*10^#-1)/3 ] &]
%Y Cf. A002275.
%K more,hard,nonn
%O 1,3
%A _Robert Price_, Apr 14 2015
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